PLATE III.

NITRATE OF UREA, CH,N,O,,HO,NO.

Fig. 1. Crystals of nitrate of urea, formed by adding excess of nitric acid to concentrated Urine.

Fig. 2. Nitrate of urea, formed by adding a quantity of nitric acid, not sufficient to combine with the whole of the urea present.

Fig. 3. Nitrate of urea, obtained by adding a moderate quantity of nitric acid to slightly concentrated Urine in a test tube, and allowed to crystallize slowly.

Fig. 4. Obtained by adding a marked excess of nitric acid.

Fig. 5. Crystals of nitrate of urea, formed by adding only two drops of nitric acid to highly concentrated Urine.

Fig. 6. Crystals of pure nitrate of urea, obtained by dissolving some of the nitrate in water, and evaporating so that crystals may form.

passed, that it crystallizes upon the addition of nitric acid, without previous evaporation. It appears, from the observations of foreign authorities, that such examples are rarely, if ever, met with on the continent.

The ordinary test for the presence of urea, depends upon the slight solubility of the crystals of nitrate of urea in water, and the readiness with which this salt is formed when nitric acid is added to a solution containing urea.

When, however, only traces of urea are present in an animal fluid as in blood, it is better to evaporate to dryness in the first instance, and extract the dry residue with alcohol. After filtration, the alcoholic solution is evaporated to the consistence of a syrup, and, if necessary, a few drops of water added. If urea be present, the characteristic crystals will be formed upon the addition of nitric acid. By this process the urea is separated from most of the saline matters, from albumen, and other substances, which would interfere with the formation of good crystals of the nitrate.

Crystals of oxalate of urea are formed when a strong solution of oxalic acid is added to a concentrated solution containing urea.

The crystals frequently take the form of rhomboidal plates, much resembling those of the nitrate, only the angles are less acute. Many prisms, with obliquely truncated summits, are often present. Usually, however, the crystals of oxalate of urea take the form of plates, which are composed of a multitude of smaller crystals as those represented in a, b, d, Plate IV; but upon microscopical examination, the rhomboidal form of many of these component crystals will be distinctly seen.

Oxalate of urea is often prepared for the purpose of obtaining pure urca, in the manner described under urea.

PLATE IV.

OXALATE OF UREA, C,H,NO, HO,CO.

Fig. 1. Crystals obtained by recrystallizing nearly pure oxalate of urea from an aqueous solution.

a. Dendritic masses, in which the form of the crystal is not very distinct.

b. Masses of well formed crystals.

c. Perfect crystals of oxalate of urea.

Fig. 2. These crystals were obtained by evaporating healthy Urine to dryness, and extracting the residue with alcohol. The alcoholic solution was evaporated to dryness, and water added until the residue had a syrupy consistence. To this oxalic acid crystals were added, in sufficient quantity to form an oxalate with the urea present.

d. Represents the general character of the crystals of oxalate usually formed in this manner.

e. More perfect crystals.